1. Field of Invention
The invention relates to a hydraulic control apparatus and method, and more particularly, to a hydraulic control apparatus and method for a hydraulic servo unit in which an operation direction is selected to a first direction when oil is supplied from a first port and discharged from a second port, and to a second direction opposite to the first direction when the oil is supplied from the second port and discharged from the first port.
2. Description of Related Art
The servo mechanism having a first port and a second port is structured to select its operation direction to a first direction when oil is supplied from the first port and discharged from the second port, and to a second direction opposite to the first direction when the oil is supplied from the second port and discharged from the first port. The above structured servo mechanism is well known as being employed for various types of machines in an automotive field, for example, a transmission, especially a toroidal type or a belt type continuously variable transmission.
The servo mechanism is provided with a hydraulic circuit having a servo valve that functions in selecting an oil supply from a pressurized oil source between the first port and the second port, while selecting a connection with a discharge oil passage between the first port and the second port. Generally the servo valve employed for the aforementioned hydraulic circuit has two oil supply ports, one of which is connected to a pressurized oil supply port and two drain ports, one of which is connected to a discharge oil passage in response to selection of a valve spool position, which is referred to as so-called a 4-port type servo valve. An example of the hydraulic circuit including the 4-port type servo valve, which is employed for the toroidal type continuously variable transmission has been disclosed in Japanese Patent Application Laid-open No. JP-A-2002-276786.
In the structure where the 4-port type servo valve serves to control operations of the servo mechanism, when a sticking or the like occurs in the servo valve and as a result, the oil supply to one of the ports cannot be stopped, the operation of the servo mechanism in one direction cannot be stopped. In the case where the servo mechanism is mounted in a transmission for a vehicle, the aforementioned failure may not be able to control the speed ratio.